linux-kernel - Re: [RFT PATCH 1/4] usb: dwc2: Simplify and fix DMA alignment code

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Message-ID: <CAKv9HNaPJFE5if2th5gqAu+_ncPrfGHVS-sJ67Zy8jao1tJZvg@mail.gmail.com>
Date:   Sun, 1 Mar 2020 17:51:42 +0200
From:   Antti Seppälä <a.seppala@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Doug Anderson <dianders@...omium.org>,
        Minas Harutyunyan <hminas@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Boris ARZUR <boris@...bu.org>, linux-usb@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Martin Schiller <ms@....tdt.de>
Subject: Re: [RFT PATCH 1/4] usb: dwc2: Simplify and fix DMA alignment code

On Sat, 29 Feb 2020 at 18:33, Antti Seppälä <a.seppala@...il.com> wrote:
>
> On Sat, 29 Feb 2020 at 17:25, Guenter Roeck <linux@...ck-us.net> wrote:
> >
> > Sigh. It would have been too simple. Too bad I can't test myself.
> > I'd like to know if this is because URB_NO_TRANSFER_DMA_MAP is set on a
> > transfer, or because the beginning of the buffer indeed needs to be aligned
> > to the DMA cache line size on that system. In the latter case, the question
> > is why the alignment to DWC2_USB_DMA_ALIGN (=4) works. In the former case,
> > question would be why the realignment does any good in the first place.
> >
> > Any chance you can add some test code to help figuring out what exactly
> > goes wrong ?
> >
>
> Sure, I can try to help. Just let me know what code you would like to
> insert and where and I'll see what I can do.
>

So I did some further research on this and it turns out that:
 - URB_NO_TRANSFER_DMA_MAP is not set on the offending transfers so
the issue really is buffer alignment
 - DWC2_USB_DMA_ALIGN=4 "works" because in my limited testcase (usb
4g-dongle utilized via qmi-wwan) all transfers are unaligned. That is,
every urb->transfer_buffer is misaligned by 2 bytes == unaligned
 - I can fix both issues and thus make the patch work on MIPS by
modifying it like this:

-#define DWC2_USB_DMA_ALIGN 4
+#define DWC2_USB_DMA_ALIGN dma_get_cache_alignment()

 struct dma_aligned_buffer {
        void *old_xfer_buffer;
-       u8 data[0];
+       u8 data[0] ____cacheline_aligned;
 };

-- 
Antti